1. Field of the Invention
The present invention relates to a growth method of a GaN crystal, and a GaN crystal substrate, conveniently employed for the substrate and the like of a semiconductor device such as a light emitting element, an electronic element and a semiconductor sensor.
2. Description of the Background Art
A GaN crystal substrate is extremely useful for the substrate of a semiconductor device such as a light emitting element, an electronic element and a semiconductor sensor. It is required to grow a GaN crystal of great thickness in order to produce such a GaN crystal substrate efficiently.
When a GaN crystal is grown on a substrate of a type differing in the chemical composition from the GaN crystal (for example, sapphire substrate, SiC substrate, and the like) (such growth is referred to as hetero growth), the mismatch in the lattice constant between the different-type substrate and GaN crystal will cause higher dislocation density of the GaN crystal to reduce the crystallinity. In addition, complicated steps such as formation of a buffer layer, formation of a mask layer and the like were required in order to reduce the dislocation density and improve the crystallinity of the GaN crystal to be grown. It was therefore difficult to grow a GaN crystal efficiently.
When a GaN crystal is grown on a GaN substrate that is identical in type with the chemical composition of the GaN crystal (such growth is referred to as homo growth), there was a problem that a crack may be generated in the GaN crystal. The generation of a crack is noticeable particularly in the case where the thickness of the homo-growing GaN crystal is 1 mm or more.
Japanese Patent Laying-Open No. 2005-200250 (hereinafter, referred to as Patent Document 1) discloses a method of growing a nitride semiconductor crystal that is uniform in quality and of high crystallinity by surrounding the growth face edge portion and side face portion of the growing crystal with a cover located apart by a predetermined distance, when a nitride semiconductor crystal is to be grown on a nitride semiconductor substrate, to prevent abnormal growth that occurs partially at the edge region of the growth face and side face of the growing crystal. However, it was difficult to suppress generation of a crack during the growth of a GaN crystal even by the method disclosed in Patent Document 1.